It is common that the tools which are supported by an industrial robot have to be supplied with energy, pressure medium, coolant, etc. This supply takes place via a plurality of connections, which may be formed as electric cables, hoses, pressure tubes or the like. In manipulators there is a general need of ensuring the supply to the tool during all the movements of the manipulator for a long period of time.
From EP 0 144 602, an industrial robot is previously known, in which the supply connections to the tool are drawn from a fixed external point in a freely suspended loop to the stand of the manipulator. From the stand, a bundle of connections run to the tool through holders fixed to the arms of the manipulator. The bundle of connections is arranged to run in loops between the holders, whereby the loops ensure that the bundle of connections may follow all the movements of the manipulator. The bundle of connections is thus subjected to a great number of bendings in various directions and with different radii of bending. These unfavorable bendings successively lead to fatigue breakdown, ultimately resulting in rupture of the connections.
A special problem arises when the connections, which are usually joined into a bundle or surrounded by a protective hose, are to be brought to run in parallel with, thus conforming to, a longitudinally rotatable robot arm. In such contexts it is common for the arm to be able to rotate, from an initial position, more than half a turn in both directions. The bundle of connections must thus run along the envelope surface of the robot arm. When the arm is in its neutral position, the necessary length is equal to the length of the arm. When rotating half a turn, however, the necessary length increases. The increase corresponds to the fact that the bundle of connections, during rotation, must be laid half a turn around the envelope surface of the robot arm. This distance constitutes half the circumference of a circle with a radius defined by the distance between the axis of rotation and the center of the bundle of connections. Calculation using the Pythagorean theorem shows that the necessary length of the bundle of connections is between 20 and 50% longer than the arm itself. During rotation of the robot arm, the bundle of connections is also subjected to a rotation around its own axis.
A common way of compensating for the length of the bundle of connections necessary for the rotation is to allow the bundle to run through two holders between which the bundle of connections extends in a loop. One holder is then fixed to the rotatable robot arm whereas the other holder is fixed to the bearing housing, in which the robot arm is journalled. The above-mentioned problem with fatigue breakdown exists also in this case. Another problem with the described embodiment is that the loop reduces the mobility of the robot and that there is a risk that the loop gets stuck somewhere, or is quite simply cut off.